1. Field of the Invention
The presently disclosed invention is related to radiometers, and, more particularly, to radiometers which employ a dual polarization correlation technique.
2. Description of the Prior Art
As is known in the prior art, radiometers are passive devices which rely on the natural black body or gray body electromagnetic radiation from a target to detect signals of thermal origin. Black body radiators are those radiators having an emissivity equal to unity while gray body radiators have an emissivity less than unity, but greater than zero. Generally, thermal emission, which is determined by absolute temperature and emissivity, is the dominant contribution to the natural radiation of the target. However, reflected and scattered radiation from other thermal emitters such the sun and the local terrain may also be a significant factor. Typically, radiometers are responsive to signals included in the range of one micron wavelength (1.mu.) to one meter wavelength (1 m). However, signals in the optical ultraviolet or other regions of the electromagnetic spectrum are also of interest.
A variety of types of radiometers have been developed in the prior art. The most common type of microwave radiometer is a Dicke radiometer which contains a radio frequency switch that selectively connects the radiometer receiver between an antenna and a reference load having a known absolute temperature. A multiplier synchronously switches the detector output to an integrator. The switching rate for the Dicke radiometer is selected to lie above the spectrum of receiver channel gain fluctuations with rates between 20 and 1,000 hertz being commonly employed. The continuous comparison of the antenna output with the matched load provides a modulation of the antenna input signal when a target signal is present while noise generated within the radiometer receiver remains unmodulated. However, switching necessarily degrades the theoretical sensitivity of a radiometer relative to total power performance. Furthermore, in the region of millimeter wavelengths, the development of a suitable switch having acceptably low insertion loss and sufficient isolation over a wide bandwidth is difficult to accomplish.
In order to circumvent the need for a wide band, radio frequency switch, a correlation radiometer was subsequently developed. In the correlation radiometer known in the prior art, thermal signals were processed through two parallel receiver channels and then correlated with each other. The basis of operation is that the signal is identical in each channel and, therefore, correlated, and, furthermore, the internal noise developed within the two channels is independent, and therefore, uncorrelated. Accordingly, in the correlation process, the correlated signals would be distinguished from the uncorrelated noise of the two receiver channels. In the case of superheterodyne receivers, the receiver channels would consist of separate mixers and intermediate frequency (IF) amplifiers with the mixers excited from a common local oscillator. This prior art dual channel correlation radiometer provided an inherent 1.5dB sensitivity improvement over a Dicke radiometer in addition to the improvement resulting from the elimination of the insertion loss of the switch.
The advantages of the correlation radiometer are however, not obtained without encountering some difficulties. In order that the assumption that the noise of the two receiver channels is uncorrelated be valid, it is essential that the two channels be isolated. That is, the noise from one receiver channel should not enter the other receiver channel. Furthermore, in the superheterodyne receivers, it is important that the noise sidebands from the common local oscillator do not produce a correlated contribution to the noise in each receiver channel. The well known techniques of employing balanced mixers and/or designating the radiometer to a high intermediate frequency can be used to reduce the local oscillator noise to limit the effect of the local oscillator noise on the receiver noise figure. However, these measures may be inadequate since the correlated local oscillator noise contribution to the receiver channel noise signals may, nevertheless, be comparable or large in comparison to the signal of interest.
In the prior art, the mutual isolation of the input signals to the two receiver channels requires the use of a hybrid junction as the input signal power divider and/or incorporation of ferrite isolators at the input port of each channel. The ferrite isolators are approximately as difficult to develop as the RF switches employed with the Dicke radiometers. Furthermore, these ferrite isolators typically have an insertion loss comparable to the ferrite switch so that the improvement in sensitivity of the dual channel correlation radiometer, which is one of the principal advantages of the device, is negated. As used in the prior art, a hybrid junction which was the preferred solution for the isolation requirement, introduced a fourth port which was terminated to maintain isolation between the separate channels. Unless the termination of the fourth port was maintained at a temperature of absolute zero, it introduced a correlated noise contribution to each receiver channel, thereby degrading the sensitivity of the radiometer. In the prior art, this effect was minimized by maintaining the termination at a cryogenic temperature or, alternatively, by connecting the fourth port with an antenna directed to a cold sky region.
Accordingly, it was recognized that a radiometer which could avoid the isolation problems of the dual channel correlation radiometer and maintain the improvement in sensitivity over the Dicke radiometer would be desirable. Furthermore, it was also appreciated that a radiometer having additional sensitivity and/or discrimination capability would be very useful.